1. Field of the invention
This invention relates to a toner for developing electrostatic images, and an image forming method, used in electrophotography, electrostatic recording, electrostatic printing or the like.
2. Related Background Art
It is hitherto well known to form an image on the surface of a photoconductive material by an electrostatic means and develop the image. A number of methods are known therefor, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748. In general, copies or prints are obtained by forming an electrostatic image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic image by the use of a toner to form a toner image, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by heat, pressure, heat and pressure, or solvent vapor. The toner which has not transferred to and has remained on the photosensitive member is cleaned by various means, and then the above process is repeated.
As methods for developing electrostatic images, it is common to use two-component development, employing a blend of toner and carrier, or one-component development, employing only a magnetic toner or only a non-magnetic toner. In either case, the toner is triboelectrically charged by means of a triboelectric charging material or member such as a carrier, a sleeve or a blade to obtain charge necessary for the development. In that case, it is known to add a charge control agent to the toner so that the desired charge quantity can be obtained. For example, as those for positively charging the toner, Nigrosine dyes, quaternary ammonium salts, polyamine resins and so forth are known in the art. As those for negatively charging the toner, salicylic acid metal complexes, dialkylsalicylic acid metal complexes, metal-containing azo dyes and so forth are known in the art. When two-component type developers are used, there is a method in which carrier particle surfaces are coated with a charge controllable material to control the changeability of the toner.
Even such a charge-controlled toner, however, may show an increase in charge quantity of the toner to cause a decrease in image density when continuously used for a long time or when continuously used in an environment of low humidity. To settle this problem, methods are proposed in which a low-resistivity material is incorporated into toner. For example, such proposals are disclosed in Japanese Patent Applications Laid-open No. 62-184473, No. 63-36268, No. 63-146048, No. 1-273056, No. 3-68956, No. 3-68957, No. 3-68958, No. 4-24646 and No. 4-124678 and Japanese Patent Publication No. 1-49941. Although the methods disclosed therein can be effective in an environment of low humidity, they have not worked well for maintaining a sufficient charge quantity in an environment of high humidity and cause fog or a decrease in image density in some instances.
To cope with this problem, the method proposed in Japanese Patent Application Laid-open No. 5-341558 employs barium sulfate whose particle surfaces are coated with antimony-doped tin oxide, and is a method that has overcome the above problems.
However, even in the above method, the tin oxide coated on the particle surfaces of barium sulfate may come off when the toner is continuously used for a long time in a developing method where a relatively great force is applied to the toner by an elastic coating blade or the like or when the toner is used in a high-speed process, resulting in lowering of the performance of the toner and also bringing about a problem that the tin oxide having come off contaminates the developing sleeve, coating blade and carrier particle surfaces. This is liable to cause problems of decrease in image density and an increase in fog.
In general, when a fine powder contains a doped low-resistivity material, electrical resistance greatly depends upon temperature so that charge quantity of the toner may be unstable to cause decrease in image density and increase in fog.